I had a look at that writeup and they never said a secondary vehicle was involved in the accident so like Tbone mentioned, unless the tractor went off course and ran into something as it appears to have stopped right in its lane, that the tractor couldn't take the jarring. Perhaps the force drove the front end up off the ground and broke in half on its way down, hard to say.
It would appear that the car, in a last second move to avoid contact, moved to the left, which made contact with the baler in the left rear. This caused the front of the baler to move left and forward, and probably put pressure upward on the tractor rear end causing the bolts to break and therefore split the tractor. No secondary collision. Wouldn't have thought the tractor would have broke like that in that sort of collision, but those are cheap built units. Car looks to have taken the impact pretty well, considering.